All Windows above XP are all awful for input lag, I have no idea why and have been trying to find an explanation since Vista came out. I don't think I've tried game mode on Windows 10 so it could be interesting to test.
I even run a completely stripped dedicated version of XP just for WA, with basically nothing but sound, graphics, and network, and I can boot into it straight to WA in about 10 seconds.
I've never played much with the other graphics modes then D3D9 shader, is CPU really less latency? Did you test it by feel? I always try to get it as low as I can in general by changing the BIOS settings for least latency, stripping the OS and changing kernel parameters, misc reg tweaks, nvidia settings, running PS/2 on the keyboard if available, using good mouse with overclocked sensor and low latency switches + weight reduction, using a CRT monitor with 150hz / whatever res - refresh suits the game best. For WA, all this is sort of a waste if your keyboard keys are 2x slower in response than another keyboard, so I "modded" a second keyboard just for rope key of which I tap the bare membrane.
Don't get me wrong, in terms of feel my setup feels amazing. The mouse feels like it's mechanically connected to my monitor (being a CRT) and everything is completely instant and smooth as glass, but I do it just to take hardware / software limitations out of the skill equation for self satisfaction. Most people who are miles better than me play on a shitty laptop or don't even think about latency or whatever.